1. Field of the Invention
The invention relates to a drawing technique wherein an optical fiber preform (hereinbelow, simply written "preform") is heated by heating means and one end of the preform in the fused state is drawn and finished to form an optical fiber. More particularly, it relates to a drawing apparatus for producing an optical fiber of predetermined uniform fiber diameter.
2. Brief Description of the Prior Art
Research of optical fibers has rapidly progressed for several years, and the technology has reached the stage at which disclosures of successful trial manufactures of optical fibers exhibiting very low transmission losses of several dB/km have been published one after another. At the present time there exists the prospect of a low transmission loss, and, as a result, a broad band optical transmission line, easy connection between the optical fibers, etc., have been sought. One of the important keys to a broad band, loss due to the connection, etc., is uniformity of the diameter of the optical fiber. The uniformity of the diameter of the optical fiber depends on an apparatus for drawing the optical fiber.
Heretofore, the drawing of the very low loss optical fiber has been carried out with an apparatus as shown in FIG. 1. A preform 1 having an outside diameter D is inserted at a constant speed v.sub.p into a furnace tube 3 heated by a heating source 2. One end of the preform heated and fused is pulled out, and wound around a drum 6. Subsequently, while under control of a motor controller 8, a motor (not shown) is driven to rotate the drum 6. As optical fiber 9 is drawn at a constant speed v.sub.f, it is taken up so as to attain an outside diameter of predetermined value d.
In this case, the diameter of the optical fiber 9 is detected by a detector 4 and is indicated on a fiber diameter measuring instrument 5. Where a fluctuation has arisen in the diameter of the optical fiber 9, fiber diameter control is conducted in such a way that an anlog output of the fiber diameter measuring instrument 5 is delivered to a control circuit 7. This is compared with a reference voltage corresponding to the predetermined fiber diameter, and the resultant output signal of the control circuit 7 is fed back to the motor controller 8, so that the take-up speed v.sub.f of the drum 6 is changed.
The inventors have studied the prior-art method from various points, and have investigated the mechanism of the fluctuation of fiber diameter. As the result, it has been revealed that factors for the fluctuation of the fiber diameter may be broadly classified into the following two:
(1) Fluctuations in the preform fusing temperature attributed to variations in air current A which flows upwards through the interior of the furnace tube 3 (i.e., attributed to disturbance).
(2) Variations in the outside diameter of the preform.
In particular, the fluctuation of the fiber diameter due to the factor (1) occurs frequently during the drawing. The fiber diameter fluctuations ascribable to the factor (1) amount to .+-. several % to .+-. several tens %. Moreover, the time constant at the fiber diameter fluctuation is on the order of a second. Therefore, even when the fiber diameter is controlled by changing the tape-up speed of the drum, it is difficult to suppress the fiber diameter fluctuation to below .+-. 2 to .+-. 3%.
Where the preform has outside diameter variations of about .+-. 3.5%, the fiber diameter fluctuations are about .+-. 7% even when the fiber diameter control is executed by prior art techniques.